The need to reduce the cost and size of electronic equipment has led to a continuing need for ever smaller filter elements. Consumer electronics such as cellular telephones and miniature radios place severe limitations on both the size and cost of the components contained therein. Many such devices utilize filters that must be tuned to precise frequencies. Hence, there has been a continuing effort to provide inexpensive, compact filter units.
One class of filter element that has the potential for meeting these needs is constructed from acoustic resonators. These devices use bulk longitudinal acoustic waves in thin film piezoelectric (PZ) material. In one simple configuration, a layer of PZ material is sandwiched between two metal electrodes. The sandwich structure is suspended in air by supporting it around the perimeter. When an electric field is created between the two electrodes via an impressed voltage, the PZ material converts some of the electrical energy into mechanical energy in the form of sound waves. The sound waves propagate in the same direction as the electric field and reflect off of the electrode/air interface.
At the mechanical resonance, the device appears to be an electronic resonator; hence, the device can act as a filter. The mechanical resonant frequency is that for which the half wavelength of the sound waves propagating in the device is equal to the total thickness of the device for a given phase velocity of sound in the material. Since the velocity of sound is many orders of magnitude smaller than the velocity of light, the resulting resonator can be quite compact. Resonators for applications in the GHz range may be constructed with physical dimensions less than 100 .mu.m in diameter and few .mu.m in thickness.
Prior art resonators have suffered from a number of problems. The first problem is the inability to tune the resonator without lowering the Q of the resonator. As noted above, the resonant frequency is determined by the thickness of the resonator. The thickness of the PZ film is fixed at fabrication; hence, the resultant resonance frequency is also fixed. Since there are variations in thickness from device to device resulting from manufacturing tolerances, some method for adjusting the resonance frequency of each device is needed.
Several methods have been suggested for altering the resonance frequency after the device has been constructed. For example, a varactor diode can be connected in series with the resonator. The varactor is then turned to change the resonance frequency of the series combination. Unfortunately, varactor diodes have inherently low Q values which, in turn, reduce the Q value of the resonator varactor combination.
A second problem with prior art resonators lies in the materials used to construct the electrodes. The acoustic path is determined by the distances between the outer edges of the electrodes, i.e., the electrode/air interface. Hence, the sound waves must pass through the electrodes as well as the PZ material. As a result, the acoustic properties of the electrodes become important. In addition, the ease of fabrication of the device must also be taken into account, since the devices are fabricated by methods that are similar to those used in semiconductor device fabrication.
The most common electrode materials are aluminum and gold. These materials are preferred because of the ease of integration of the materials into the fabrication process. Aluminum metalization is commonly used in semiconductor fabrication processes; hence, its methodology is well understood. In addition, aluminum is less expensive and outperforms gold from an acoustic point of view.
Aluminum has two disadvantages. First, it is difficult to selectively etch aluminum. Selective etching is less of a concern in semiconductor fabrication than in resonator fabrication, since the aluminum is usually deposited on materials which are compatible with previously known selective etch processes. In resonator fabrication, the electrode layers are preferably created by etching. Second, aluminum has relatively high thermal elastic losses. These losses reduce the performance of the resonator.
Broadly, it is the object of the present invention to provide an improved thin film acoustic resonator.
It is a further object of the present invention to provide an acoustic resonator that may be tuned after fabrication.
It is a still further object of the present invention to provide an acoustic resonator with electrodes constructed from materials having superior properties than the prior art aluminum electrodes.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.